Geological Society. 1 ] 5 



On the conchological history of the crag, Mr. Clarke offers no re- 

 marks, partly because it did not fall within his object in writing the 

 paper, and partly because the data which he formerly collected have 

 been lost. 



Diluvium. — The diluvium of Suffolk may be divided into three 

 classes : 1 , clay ; 2, gravel j and 3, erratic blocks. 



1. Clay. — This division covers a considerable portion of the 

 county, and extends into Norfolk, Cambridgeshire, and Essex, rising 

 to a considerable elevation in High Suffolk, and attaining near Cromer, 

 a thickness of 400 feet. A considerable portion of the clay is of a 

 yellowish hue, but the greater part is blue j and both varieties contain 

 chalk pebbles, sometimes disposed in layers but more commonly di- 

 spersed, a character by which the diluvial may be distinguished from 

 the London or plastic clay. It is difficult to determine the origin of 

 this argillaceous deposit ; but the author is inclined to think, that the 

 yellowish portion may have been derived from the plastic clays, and 

 the blue from the clays below the chalk. Fragments of coal have 

 been found in the diluvium at Lavenham, also fragments of mica slate 

 containing garnets and tourmaline. Specimens of a similar rock were 

 obtained by Mr. R. C. Taylor, at Cromer, with masses of granite, por- 

 phyry, trap, oolites, &c* At Baliingdon Hill near Sudbury, Mr. Brown 

 has procured thirty varieties of primary, secondary, and tertiary rocks. 

 Comparatively few flints occur in the clay. At Ickworth a beautiful 

 specimen of the Dudley trilobite was obtained in making a drain : 

 and at various other localities, numerous species of tertiary and 

 secondary fossils abound. 



It is inferred that the clay contains cavities, as streams of noxious 

 air occasionally issue from fissures. 



2. Gravel. — The gravel is less generally diffused than the clay, and 

 is considered by Mr. Clarke to have been partly deposited at a distinct 

 period. In some cases, it consists merely of unrolled flints, left in 

 situ by the dissolution of the chalk ; in others, large masses of flint 

 slightly mixed with chalk pebbles are imbedded in sand ; and occa- 

 sionally flints are intermingled with boulders of various dimensions, 

 and sometimes unknown origin. Many of these extraneous fragments, 

 he thinks, may have been washed out of the clay j and he shows that 

 the river valleys have been excavated through both the clay and the 

 gravel. With respect to the relative proportional quantity of each 

 ingredient, chalk flints are said to be the most numerous, primary and 

 transition rocks the next in abundance, and secondary and tertiary 

 the fewest in number; the absence of the two latter being explained 

 by their inferior hardness. 



3. Erratic blocks. — These occur in great abundance, and occasion- 

 ally of vast size. They are sometimes found in the river valleys, and 

 sometimes on the level platforms and hills. They agree in lithological 

 characters with the smaller fragments of the gravel, and are con- 

 sidered to be of the same diluvial origin ; but are so conspicuous as 

 to deserve a distinct notice. 



Lacustrine deposits. — Under this head the author alludes to the bed 

 containing freshwater shells, discovered by Mr. Charlesworth and Mr. 

 • See Phil. Mag. and Annals, N.S. vol. i. p. 346. 

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